A multilayer film filter transmits or blocks light having a specific wavelength, or changes light intensity regardless of wavelengths. Multilayer film filter chips are obtained by alternately forming low refractive index films such as SiO2 and high refractive index films such as TiO2 or Ta2O5 onto the top surface of a substrate by sputtering or evaporation, and then by dividing with a dicing cutter.
The conventional glass substrate for a multilayer film filter has a large thermal expansion coefficient. The reason for this is to reduce the amount of shift in filtering properties with temperature changes (hereinafter referred to as wavelength shift). An example of glass of this type is disclosed in Japanese Patent Laid-Open Application No. 2001-48584.
FIGS. 1 and 2 show changes in filter properties with temperature changes. In many filters, as shown in FIG. 1, filtering properties 21 make a large shift in the positive direction with a temperature increase so as to change into filtering properties 22. A large amount of shift means that the filter properties change greatly with temperature changes. Therefore, when the wavelength shift is large, the filter can be used as a filter having required properties only in a narrow temperature range. In other words, a filter making wavelength shift closer to 0 can be used as a filter having required properties in a wider temperature range.
It is known that the larger the thermal expansion coefficient of the glass for a multilayer film filter than the thermal expansion coefficient of the multilayer film is, the longer the wavelength shift which occurs in the negative direction becomes. On the other hand, the thermal expansion coefficient of the glass used as a substrate is generally around 100×10−7/° C. in the temperature range of 50° C. to 150° C., and the wavelength shift often has a positive value. When the wavelength shift occurs in the positive direction, a substrate having a thermal expansion coefficient not lower than 100×10−7/° C. is required for having the shift value close to 0.
In a case where more complicated properties are required as the filter properties, it is necessary to increase the number of layers in the multilayer film, thus leading to an increase in film thickness as a whole. In general, in forming a multilayer film onto the glass for a multilayer film filter, the amount of wavelength shift increases with increasing thickness of the multilayer film. From this reason, the thermal expansion coefficient of the glass for a multilayer film filter is preferably larger than that of the conventional glass material. Using such glass having the larger thermal expansion coefficient makes it possible to obtain a filter having a small shift as shown between filter properties 31 at a low temperature and filter properties 32 after a temperature rise in FIG. 2.
On the other hand, it is possible to maintain light transmittance and to improve the thermal expansion coefficient by crystallizing the glass partially to make small particles which have a large thermal expansion coefficient for a multilayer film filter. Such partially crystallized glass is sometimes used as the glass for a multilayer film filter. Even in that case, the highest thermal expansion coefficient of the partially crystallized glass is about 125×10−7/° C. in a temperature range of 50° C. to 150° C. so far, and there are cases where the wavelength shift of the multilayer film filter cannot be fully reduced.